Method for producing ultra-high purity, optical quality, glass articles

ABSTRACT

A method for consolidating a green body is disclosed which involves: (1) drying and partially sintering the green body at a temperature above about 1000° C. and in an atmosphere containing chlorine; (2) fully sintering the green body under vacuum at a temperature above about 1720° C.; and (3) hot isostatic pressing (&#34;hipping&#34;) the green body at a temperature above about 1150° C. and at a pressure above about 100 psig. The process produces glass articles which have a low water content and are essentially bubble free. 
     This is a continuation of co-pending application Ser. No. 07/271,709 filed Nov. 16, 1988, now abandoned which is a divisional application of application Ser. No. 07/052,619, filed May 20, 1987 now U.S. Pat. No. 4,789,389.

CROSS REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for producing ultra-high purity,optical quality glass articles. More particularly, as described in fulldetail below, the invention involves: 1) using a sol-gel process to formfused silica granules, 2) preparing a green body from the granules, 3)purifying and consolidating the green body, and 4) subjecting theconsolidated green body to hot isostatic pressing ("hipping") to producethe desired finished product.

2. Description of the Prior Art

Numerous investigators have attempted to apply the sol-gel technique tothe production of optical quality glass products.

For example, Matsuyama et al., UK Patent Application No. GB 2,041,913,describes a gel casting method for producing "mother rods" from whichoptical waveguide fibers can be prepared wherein a solution of a siliconalkoxide is formed, allowed to gel so as to produce a porous preform,dried, and then sintered at a temperature below its melting temperatureto produce the mother rod. The application describes a three stepsintering process in which an atmosphere of oxygen and helium is used upto a temperature of 700° C., an atmosphere of chlorine and helium isused between 700° C. and 1000° C., and an atmosphere of just helium isused above 1000° C. As acknowledged in this application, drying the gelwithout cracking is difficult and can take as long as 10 days.

U.S. Pat. No. 4,419,115 to David W. Johnson, Jr., et al., describes asimilar process for producing glass articles wherein fumed silica ismixed with a polar liquid to form a first sol, the first sol is gelledto form a first gel, the first gel is dried, heated to a temperature inthe vicinity of 750°-850° C., cooled, redispersed in a polar liquid toform a second sol, the second sol is gelled to form a second gel, thesecond gel is dried, and the dried second gel is sintered to form theglass article.

The Johnson et al. patent states that the heating of the first gel to750°-850° C. does not result in densification of the gel material.Specifically, the patent states that until final sintering, the BETsurface area of its silica material remains essentially the same as thatof fumed silica. With regard to sintering, the patent states that ahelium atmosphere, which optionally contains chlorine, or a vacuum canbe used during this step. Significantly, the patent employs the heliumplus chlorine approach, and not the vacuum approach, in each of itsexamples. In practice, the process of the Johnson et al. patent, likethe process of the Matsuyama et al. application, has been found to besubject to gel cracking problems.

In addition to the foregoing, sol-gel casting processes have also beendescribed in Hansen et al., U.S. Pat. No. 3,535,890, Shoup, U.S. Pat.No. 3,678,144, Blaszyk et al., U.S. Pat. No. 4,112,032, Bihuniak et al.,U.S. Pat. Nos. 4,042,361, and 4,200,445, and Scherer, U.S. Pat. No.4,574,063, European Patent Publication No. 84,438, and Scherer et al.,"Glasses from Colloids", Journal of Non-Crystalline Solids, 63:163-172(1984).

In particular, the Hansen et al. patent relates to a process in which anaqueous solution of colloidal silica particles is formed, dried toproduce a gel, and the gel is sintered in a three step process, thefirst step comprising heating the gel to around 600° C. in a vacuum, thesecond step comprising flushing the gel with chlorine gas to removebound water, and the third step comprising sintering the gel undervacuum by raising its temperature to 1200° C. The patent acknowledgesthe gel's high sensitivity to cracking during the drying process andstates that drying times on the order of many days or weeks are neededto overcome this problem.

The Shoup patent, as well as the Blaszyk et al. patent, relate to aprocess in which gels are formed from soluble silicates, such as, alkalisilicates. The dried gels can be used, for example, as filters, solidsupports for catalysts, and the like, or can be consolidated into asolid glass body at temperatures ranging from 600°-1700° C. The gelsproduced by the soluble silicate technique are generally stronger thanthose produced by other sol-gel procedures. This makes crack-free dryingof the gel easier and also facilitates the production of large castings.Alkali silicate solutions, however, contain significant amounts of iron.Accordingly, a leaching step is required if high purity glass is to beproduced. Leaching is also generally required if the final product is tobe alkali-free. In one set of examples, the Shoup patent comparesconsolidating a gel in air with consolidating a gel under a reducedpressure of 20 mm. In some cases, the reduced pressure resulted in aconsolidated product which did not include bubbles: in other cases,bubbles still remained.

The Bihuniak et al. patents describe processes for densifying fumedsilica and other fumed metal oxides by forming a sol, drying the sol toform fragments, and densifying the fragments by calcining them at1150°-1500° C. Thereafter, the densified material can be milled, e.g.,to an 8 to 10 micron average particle size, suspended in a castingmedium, slip cast to form a porous preform, and fired to produce thedesired finished product.

Because it employs fumed silica, the Bihuniak et al. process is moredifficult to perform than the process of the present invention. Forexample, it is relatively difficult to form gels from fumed silica, andas acknowledged in the Bihuniak et al. patents, once formed, gels madefrom fumed silica tend to break up into large chunks, rather than smallparticles, as is desired. Further, extensive pollution abatementequipment is required to produce fumed silica since such productioninvolves the creation of hydrochloric acid.

In addition, densified silica particles made from fumed silica tend tohave higher impurity levels than the densified silica particles producedby the process of the present invention. These higher impurity levelsare due in part to the fact that impurities, including trace amounts ofradioactive materials, are introduced into the silica during the fumingprocess.

The higher impurity levels also arise from the fact that densificationof particles made from fumed silica gels requires higher temperaturesthan densification of particles formed from gels prepared in accordancewith the present invention, i.e., densification of particles made fromfumed silica gels require temperatures above, rather than below, 1150°C. Such higher temperatures generally mean that metal-containingfurnaces must be used to perform the densification. The use of suchfurnaces, in turn, means that the silica particles will be exposed toand thus will pick up metal ions which are released from the walls ofthe hot furnace. In addition to the purity problem, the need to generatehigher temperatures to achieve densification is in general undesirable.

The Scherer references describe forming a gel from fumed oxides in anon-aqueous medium, e.g., an organic medium, drying the gel, exposingthe dried gel to vacuum for a few hours and heating the gel in oxygen toremove residual organic constituents, and then sintering the gel in ahelium or helium plus chlorine atmosphere.

As with various of the sol-gel techniques described above, the gelsproduced by the Scherer technique are relatively fragile and thus mustbe carefully handled to avoid cracking. Also, as is typical of processesin which gels are sintered, gels prepared in accordance with the Schererprocess undergo a linear shrinkage of approximately 40% upon sintering.Such a shrinkage level makes it relatively difficult to cast complexshapes and also leads to relatively high levels of gel fracture duringsintering. In addition to the foregoing, because the Scherer processuses fumed silica, it suffers from the impurity and pollution controlproblems associated with the fuming process (see discussion above).

The use of hot isostatic pressing ("hipping"), as well as other pressingtechniques, to compress gas bubbles in vitreous materials has beendescribed in a number of references. See Rhodes, U.S. Pat. No.3,310,392, Bush, U.S. Pat. No. 3,562,371, Okamoto et al., U.S. Pat. No.4,358,306, and Bruning et al., U.S. Pat. No. 4,414,014 and UK patentapplication No. 2,086,369. The Bush patent, in particular, disclosesforming a green body, sintering the body in a vacuum, and thensubjecting the consolidated body to isostatic pressure at a temperatureequal to or greater than the sintering temperature.

SUMMARY OF THE INVENTION

In view of the foregoing state of the art, it is an object of thepresent invention to provide an improved process for producing opticalquality, high purity, glass articles. In particular, it is an object ofthe invention to provide a process for producing such articles whichinvolves the sintering of a porous silica body but avoids the cracking,shrinkage and purity problems encountered in prior art processes cf thistype.

With regard to products, it is an object of the invention to provideultra-pure fused silica granules ("artificial sand") which can be usedin a variety of conventional ceramic forming processes, such as, powderpressing, extrusion, slip casting, and the like, to produce greenbodies. It is an additional object of the invention to produce glassarticles of complex shapes which have higher purities, more uniformtransmittance characteristics, and smaller index of refractionvariations, i.e., better homogeneity, than similar articles produced byprior art techniques. It is a further object of the invention toeconomically produce optical waveguide fibers which have transmissioncharacteristics equivalent to optical waveguide fibers produced by moreexpensive techniques.

In accordance with the invention, the foregoing and other objects areachieved by using the following method steps to produce fused silicaglass articles:

(a) preparing a solution which contains at least one silicon-containingorganic compound having the formula Si(OR)₄ or SiR(OR)₃, where R is analkyl group;

(b) polymerizing the silicon in the solution to form a SiO₂ gel;

(c) drying the gel at a rate which causes the gel to fragment intogranules having a mean particle size less than about 1 millimeter;

(d) sintering the granules at a temperature less than about 1150° C.,the density of the granules after sintering being approximately equal totheir maximum theoretical density:

(e) forming a green body from the sintered granules;

(f) drying and partially sintering the green body in a chamber by:

(i) introducing oxygen into the chamber to reduce the level of organicmaterials associated with the green body: and

(ii) raising the temperature of the chamber to above about 1000° C.,e.g., to 1150° C., while introducing chlorine into the chamber and/orpurging the chamber with an inert atmosphere and/or subjecting thechamber to a vacuum to reduce the level of water associated with thegreen body;

(g) fully sintering the green body in a chamber by raising thetemperature of the chamber to a temperature above about 1720° C. whilepurging the chamber with helium or preferably applying a vacuum to thechamber: and

(h) hot isostatic pressing the fully sintered green body in a chamber byraising the temperature of the chamber to above about 1150° C. andintroducing an inert gas into the chamber at a pressure above about1,000 psig and preferably above about 15,000 psig.

As discussed in detail below, for certain applications, subgroupings ofthe foregoing eight process steps can be employed. For example, steps(a) through (d) can be used to produce high purity, fused silicagranules, which in themselves are useful articles of commerce.Similarly, green bodies produced from materials other than the glassgranules of the present invention can be consolidated into bubble andwater free glass articles through the use of steps (f) through (h).

Particular process steps can also be omitted depending on the specificconditions used and the purity requirements of the final product. Forexample, chlorine treatment may not be required in step (f) if thefinished product does not have to have a low water content. Othermodifications of this type are discussed below in connection with thedescription of the preferred embodiments of the invention.

Unlike prior art techniques which have employed sol-gel technology, theforegoing method provides a practical procedure for commerciallyproducing ultra high purity, optical quality glass articles. The successof this technique is due to a number of factors. In the first place, thetechnique of the present invention does not use sol-gel technology toform a green body. Rather, sol-gel technology is used to produceartificial sand, and then conventional techniques, e.g., slip casting,are used to form a high density green body whose shape and size are nearto that of the final product. In this way, the two main problemsassociated with sol-gel technology, i.e., gel cracking during drying andgel shrinkage upon sintering, are avoided.

Indeed, the present invention, rather than trying to prevent gels fromcracking during drying, affirmatively uses the cracking phenomenon tocreate its artificial sand. Thus, rather than drying gels slowly, as wasdone in the prior art, the gels of the invention are dried rapidly sothat they fragment into small granules.

To achieve this fragmentation, the gels of the invention are made fromsilicon-containing organic compounds, e.g., fromtetraethylorthosilicate, rather than from fumed silica. Gels made fromsuch organic compounds have smaller pore sizes than gels made from fumedsilica. Accordingly, during drying, greater stresses are generated inthese gels, and thus smaller particles are produced when the gels crackduring drying. Depending on the application, these small particles caneither be used directly or, if desired, can be milled to smaller sizesusing conventional techniques and equipment.

In addition to their fragmentation characteristics, the use of gels madefrom siliconcontaining organic compounds, rather than fumed silica, isalso advantageous for various other reasons. First, as discussed above,it is in general easier to make gels from silicon-containing organiccompounds than from fumed silica. Further, because of the differences inthe way they are manufactured, higher purities can be achieved forsiliconcontaining organic compounds than for fumed silica. Also, lesspollution is generated during the manufacturing of the organic compoundsthan during the manufacturing of fumed silica.

In addition, it is easier to maintain high levels of purity when workingwith gels made from silicon-containing organic compounds than whenworking with gels made from fumed silica. Specifically, because gelsmade from organic compounds have higher densities, water contents, andsurface areas than gels made from fumed silica, granules produced fromorganic compound gels can be sintered at lower temperatures thangranules produced from fumed silica gels. Lower sintering temperaturesmean that it is easier to keep the sintering environment free fromcontaminants. In particular, the lower sintering temperatures mean thathigh purity, silica-based reactors, as opposed to metallic or ceramicZrO₂ furnaces, can be used to perform the sintering.

In addition to providing high purity, fused silica granules, the methodof the invention also carries the high purity level of the granulesthrough to the final product and, at the same time, produces a finishedproduct having excellent optical properties. In particular, the oxygenand chlorine treatments during the drying of the green body specificallyreduce the levels of water and organic matter in the finished product.In addition, the use of the preferred vacuum sintering means that anybubbles or similar defects which are created during sintering will inessence be empty voids. These empty spaces can be easily closed duringhipping.

As described in detail in Example 2 below, it has been found that bymeans of the invention, finished products of complex shapes, such as,optical domes, antenna windows, sight glasses, aerospace viewports,lenses, prisms, mirrors, etc., can be readily produced which haveequivalent or better optical properties than similar products producedby other techniques. In particular, the products have been found to havehigher purities, smaller index of refraction variations (betterhomogeneities), and more uniform transmittance characteristics from theultraviolet through the infrared than similar commercial products whichhave heretofore been available. In addition, as illustrated in Example3, the method of the invention can be used to produce low loss, opticalwaveguide fibers. Significantly, in accordance with the invention,production costs for such fibers can be reduced.

The principles of the invention, as well as its preferred embodiments,are explained and illustrated by the accompanying figures and theexamples presented below. These figures and examples, of course, are forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show the appearance of the silica granules of the presentinvention before and after sintering, respectively. The numbers on thescales shown in these figures represent centimeters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, the present invention involves producing glassarticles by: 1) using a sol-gel process to form fused silica granules,2) preparing a green body from the granules, 3) purifying andconsolidating the green body, and 4) subjecting the consolidated greenbody to hot isostatic pressing to produce the desired finished product.

The sol-gel process employs at least one silicon-containing organiccompound having the formula Si(OR)₄ or SiR(OR)₃, where R is an alkylgroup. Tetraethylorthosilicate ("TEOS"), having the formula Si(OC₂ H₅)₄,is the preferred silicon-containing organic compound, although otherorganosilicon compounds, including, for example,tetramethylorthosilicate having the formula Si(OCH₃)₄, andmethyltrimethoxysilane, having the formula SiCH₃ (OCH₃)₃, can be used.The silicon-containing organic compound can be partially hydrolyzed. Forexample, partially hydrolyzed TEOS sold by the Stauffer Chemical Companyunder the trademark SILBOND 40 is a suitable starting material forpreparing the gels of the present invention. In general, the use of asingle silicon-containing organic compound is preferred, althoughmixtures of such compounds can be used, if desired.

A solution of the silicon-containing organic compound is prepared andthen gelled. Preferably, the solution is an aqueous solution whichincludes an acid, such as, hydrochloric acid, as a gelation catalyst.Other solvents, e.g., organic solvents such as ethanol, can be added toimprove miscibility, and other gelation catalysts, e.g., acids such asformic and nitric acid, can be used.

In the case of TEOS, water to TEOS mole ratios in the range of 4:1 to15:1 have been found to work successfully. Theoretically, ratios as lowas 2:1 can be employed. Higher ratios produce gels having larger surfaceareas and pore sizes which are easier to dry. In some cases, for higherratio gels, drying alone has been found to sufficiently reduce the levelof organic material within the gel, so that oxygen treatment of the gelcan be omitted. The higher ratio gels, however, mean that less productis produced for the same size reaction vessel. In general, a mole ratioof 6:1 has been found to produce a reasonable amount of product whichcan be readily dried.

If the final product is to be doped, in addition to the primarysilicon-containing organic compound, the solution will also containother organic or inorganic compounds which include the dopant elements.Examples of elements which can be introduced into the solution in theform of soluble compounds include aluminum, antimony, barium, beryllium,bismuth, boron, bromine, cadmium, calcium, cerium, chlorine, chromium,cobalt, copper, europium, fluorine, germanium, iron, lanthanum, lead,lithium, magnesium, necdymium, nickel, oxygen, phosphorous, potassium,samarium, silver, sodium, strontium, tantalum, tin, titanium, uranium,vanadium, yttrium, zinc, and zirconium. One or more dopants can be used,as desired.

A particularly preferred dopant is titanium since it allows for theproduction of ultra-low expansion glasses, i.e., glasses having anexpansion coefficient of less than 0.5×10⁻⁷ cm/cm/° C. This dopant canbe conveniently introduced into the solution as a titanium ester havingthe formula Ti(OR)₄, where R is an alkyl group. Examples of titaniumesters which are suitable for use with the present invention includetetraethyl titanate having the formula Ti(OC₂ H₅)₄ and tetraisopropyltitanate having the formula Ti(OCH(CH₃)₂)₄.

The solution of the organosilicon compound and dopants, if used, ispreferably gelled in a quartz reactor or similar vessel. Gelationresults in 1) polymerization of the silicon, and 2) the production of analcohol, such as, ethanol in the case of TEOS. Typical gelation timesfor a solution pH in the range of 1-2 are in the range of 1-4 hours at atemperature of from 60° C. to 75° C. Gelation times can be reduced byapplying heat to the organosilicon solution. Also, if desired, highspeed gelation can be achieved by neutralizing the pH of the TEOSsolution through the addition cf a basic solution, such as, a 1.2%ammonium carbonate solution. Gelation times in the range of seconds canbe achieved in this way.

Once gelation has been completed, the gel is dried to 1) remove residualwater and alcohol (carbon), and 2) fragment the gel into granules havinga mean particle size less than about 1 millimeter. The drying can beconveniently performed in the same reactor used to prepare the gel, orthe gel can be removed and dried in specialized drying equipment, e.g.,in a rotary dryer or rotary vacuum dryer.

When the drying is performed in the reactor used to prepare the gel,drying temperatures above about 250° C. are preferred. At suchtemperatures, drying times on the order of 30 hours are typical. Toremove the water and alcohol, the gel is either purged with an inertatmosphere, e.g., an argon atmosphere, or subjected to a vacuum. Purgingand vacuum treatments can be used sequentially, if desired.

When the drying is performed using a rotary dryer or a rotary vacuumdryer, temperatures above the boiling point of water, e.g., on the orderof 120° C., have been found to be adequate. In the case of vacuum rotarydryers, it has been found that vacuum should not be applied to the gelat the beginning of the drying process since it causes the gel tocollapse which impedes the removal of water and organics. Vacuum,however, can be used at the end of the drying process where it does helpin the removal of water and organics. Typical drying times when either arotary or rotary vacuum dryer is used are on the order of 8 hours. Ithas been observed that smaller granules are produced by rotary dryingthan by bulk drying in the reactor in which the gel was produced.

In addition to the foregoing methods, drying can be accomplished byforming the gel in thin sheets and allowing the sheets to dry at roomtemperature. The thin sheet approach, however, is not preferred forcommercial production.

Depending on the characteristics of the gel and the particular dryingconditions employed, drying alone may reduce the level of organicsassociated with the granules to a point where charred carbon particlesdo not form upon sintering of the granules. Alternatively, the organicscan be removed by purging the gel granules with an oxygen-containingatmosphere at an elevated temperature. To avoid the production ofcharred carbon particles during this process, the rate of oxygen purgingmust be controlled so that the exothermic reaction between oxygen andorganics does not cause the temperature of the drying chamber to riseabove about 340° C.

In the case of gels formed from TEOS, depending on the H₂ O :TEOS ratio,the amount of acid catalyst used, and the specific drying protocolemployed, the dried gel granules will typically have a density ofapproximately 1.29-1.59 grams/cc, a pore size of approximately 20-100angstroms, and a surface area to mass ratio of between about 150 andabout 700 meter^(2/) gram.

For comparison, if fumed silica is used as the starting material, theresulting dried gel will have a lower density of approximately 0.31-0.88grams/cc and a larger pore size of approximately 700-1000 angstroms.Moreover, the drying of such a fumed silica gel will generally notproduce granules having a mean particle size less than one millimeter,but rather, will produce larger particles and chunks which are notwell-suited for further processing.

Once the gel has been dried and thus fragmented, the gel granules arefully sintered, i.e., they are sintered to a density which isapproximately equal to their maximum theoretical density. The sinteringprocess causes various changes in the character of the gel granules.Specifically, during sintering, the polymeric structure of the gelgranules relaxes, water is given off (condensation reaction) which, inturn, affects the granules'apparent viscosity, and the pores of thegranules collapse. Overall, the sintering process results in a dramaticdecrease in surface area to mass ratio of the granules, i.e., from the150-700 meter^(2/) gram range to a value less than 0.5 meter² /gram.

The sintering of the gel granules is conducted at a temperature of lessthan about 1150° C. This low sintering temperature allows the sinteringto be conducted in the same quartz reactor used for gelation and drying.The use of such reactor, as opposed to a metal furnace, helps maintainthe purity of the granules through the sintering procedure.Alternatively, if the gel has been removed from the original quartzreactor for drying, it can be returned to a silica-based reactor forsintering.

For comparison, to sinter a gel formed from fumed silica requirestemperatures well above 1150° C., e.g., in the range of 1250-1450° C.This difference in sintering temperatures is due primarily to the factthat the pore size for gels made from fumed silica is generally on theorder of 700 angstroms, while the pore size for gels made fromsilicon-containing organic compounds is generally less than about 100angstroms. As known in the art, sintering temperature generallydecreases with decreasing pore size. See Scherer et al., supra.

The sintering can be performed in a variety of atmospheres. For example,helium, helium/oxygen, and argon/oxygen atmospheres can be used. In somecases, a helium atmosphere has been found preferable to an argon/oxygenatmosphere. The sintering can also be performed in air.

A sintering period of approximately one hour at temperatures in the900-1000° C. range is generally sufficient to achieve full densificationof the granules. The specific temperature needed will depend on poresize of the gel. The pore size, in turn, will depend on the H₂ O:TEOSratio used to produce the gel. As illustrated in Table I, higher moleratios result in gels having larger surface areas but lower densities.Accordingly, the pore sizes for these gels are greater and thus highersintering temperatures are required to achieve full densification.

Sintering temperature is also affected by the amount of chemically boundwater associated with the granules. For example, it has been found thatif the water level in a gel has been reduced by means of a chlorine gastreatment, high sintering temperatures, e.g., temperatures 100-150° C.higher, are required to obtain full densification of the granules.

The fully-sintered granules constitute ultra-pure, artificial sand. Asdiscussed in copending application Ser. No. 052,655, now U.S. Pat. No.4816299, granted 3/28/89 which is entitled "Encapsulating CompositionsContaining Ultra-Pure, Fused-Silica Fillers" and which was filed on May20, 1987, the fully-sintered granules can be used as a filler forpotting sensitive electronic components, such as, semiconductor memorydevices. In comparison with prior art silica fillers, the granulescontain lower amounts of such radioactive materials as uranium andthorium, and thus produce less alpha particles which can interfere withthe operation of state-of-the-art electronic components. The pertinentportions of the above-referenced copending application are incorporatedherein by reference.

In accordance with the present invention, the fully-sintered granulesare used to form high density green bodies. In particular, the granulesare used as the starting material for such conventional processes asslip casting, injection molding, extrusion molding, cold isopressing,and the like. A description of these and other processes in which theartificial sand of the present invention can be used can be found insuch texts as Introduction to Ceramics, by D. Kingery, John Wiley andSons, Inc., New York, 1960, and Ceramic Processing Before Firing, G. Y.Onoda, Jr., and L. L. Hench, editors, John Wiley and Sons, Inc., NewYork, 1978, the pertinent portions of which are incorporated herein byreference. With regard to slip casting in particular, descriptions ofthis technique can be found in U.S. Pat. No. 2,942,991 and in Whiteway,et al., "Slip Casting Magnesia," Ceramic Bulletin, 40:432-435 (1961),the pertinent portions of which are also incorporated herein byreference.

For many of these processes, such as, slip casting, it is preferred toreduce the size of the granules prior to casting. This can be readilyaccomplished by a variety of milling techniques, includingvibra-milling, ball milling, jet impingement or fluid energy milling,triter milling, and the like. Combinations of these milling techniquescan also be used. If desired, the milling can be performed prior tosintering of the granules. In general, it has been found that unsinteredgranules are easier to fracture than sintered granules.

For slip casting, it has been found preferable to form the green bodyfrom a slurry which has a specific gravity greater than about 1.70grams/cc and which is composed of granules having a mean particle sizein the 10-15 micron range. Such a slurry can be conveniently producedusing a urethane-lined vibra-mill to which the granules, silica media,and water are added. Using a slurry of this type, high density greenbodies, e.g., green bodies having a porosity on the order of 20%, arereadily prepared.

For various of the other casting methods, e.g, the injection, extrusion,and pressing techniques, it is generally preferred to employ a binder inthe slurry. Such a binder can be conveniently formed by in situhydrolyzation of TEOS. By way of illustration, a slurry of the granulesof the present invention was successfully cast in a plastic mold, asopposed to a plaster of Paris mold, by adding 5 milliliters of anacid-catalyzed TEOS/water mixture (4 moles water to each mole of TEOS)to 132 milliliters of slurry. After molding, 2-7 milliliters of a basicsolution (1.2% ammonium carbonate) was added to the slurry. The basicsolution shifted the pH causing the TEOS to gel within a period of fromabout 2 to about 30 minutes, thus binding the granules together to forma strong green body, well-suited for further processing. Alternatively,commercial binders, such as those sold by the Stauffer Chemical Companyunder the SILBOND trademark, can be used.

Once formed, the green body is purified and consolidated by a two-stepprocess. In the first step, the green body is dried and partiallysintered. In the second step, the green body is fully sintered.

The drying and partial sintering step, among other things, serves toremove water from the green body which could form bubbles in the finalproduct during full sintering. To minimize contamination, this step ispreferably performed in a quartz tube furnace, although other types offurnaces can be used, if desired. When a quartz tube furnace is used,the temperatures employed are preferably kept below about 1150° C.

Drying and partial sintering are achieved by raising the temperature ofthe furnace to above about 1000° C., while introducing chlorine into thefurnace and/or applying a vacuum to the furnace and/or purging thefurnace with one or more inert gases, e.g., with argon and/or helium.The chlorine treatment, vacuum stripping and/or inert gas purgingreduces the chances that the water content of the green body will causebubbles to form during full sintering. In addition to removing water,the chlorine treatment has also been found to reduce the green body'siron, copper, and calcium levels. When the green body is formed by slipcasting, the chlorine treatments ability to strip calcium is ofparticular value since the green body tends to pick up calcium from theplaster of Paris mold.

Optionally, the drying and partial sintering step can include subjectingthe green body to an oxygen-containing atmosphere to reduce its contentof organic materials.

The oxygen treatment can be omitted if the green body includes onlyminor levels of organic material contamination. The chlorine treatmentcan be omitted in cases where the final product can have a relativelyhigh water content, e.g., in cases where the absorption characteristicsof the final product in the infrared region are not critical. When thechlorine treatment is omitted, either vacuum stripping or inert gaspurging should be performed. If desired, both vacuum stripping and gaspurging can be used sequentially. Either or both the vacuum strippingand the inert purging can be omitted when the chlorine treatment isused.

After the green body has been dried and partially sintered, it is fullysintered at a temperature above about 1720° C. Full sintering ispreferably performed in a vacuum of, for example, 1×10⁻⁵ torr.Alternatively, helium purging can be used, although this is lesspreferred since any bubbles which form in the glass during sinteringwill be filled with helium, rather than being empty, as occurs duringvacuum sintering.

The full sintering of the cast granules can be performed in, forexample, a tungsten-molybdenum furnace or a helium-filled graphitefurnace. To minimize contamination, the green body is preferablysupported on quartz cloth and monoclinic unstabilized zirconia A grain.Specifically, it has been found preferable to use monoclinicunstabilized zirconia, as opposed to stabilized zirconia, as the supportfor the green body during the sintering process. Grain of this type isavailable from Corning Glass Works, Corning, N.Y., under productdesignation Zircoa GGA.

In general, full sintering, as well as cooling of the sintered product,can be completed in about 3 hours. Thereafter, if desired, the surfacesof the consolidated green body can be cleaned with hydrofluoric acid.Also, areas of the green body which may have become deformed duringsintering, e.g., areas in contact with the quartz cloth, can be removedby grinding.

For certain applications, e.g., the production of consolidated preformsfor optical waveguide fibers, the fully sintered green body may be readyfor use without further processing. In most cases, however, it ispreferred to hip the sintered green body to collapse any bubbles whichmay have formed in the body during the sintering process.

The hipping is performed in the pressure chamber of a hipping furnace(see, for example, U.S. Pat. No. 4,349,333) by heating the chamber to atemperature greater that the annealing point of the consolidated greenbody and less than about 1800° C., while introducing an inert gas, suchas, argon, helium, or nitrogen, into the chamber at a pressure in therange of 100-45,000 psi. In practice, temperatures in the range of1150°-1740° C. and pressures in the range of 1,000-30,000 psig have beenfound suitable for collapsing bubbles and other voids in consolidatedgreen bodies produced in accordance with the present invention. Lowerpressures, e.g., pressures in the 100-1000 psig range, can also be used.

To avoid contamination of the consolidated green body during hipping, itis preferred to wrap the body in glass wool and steel foil before it isplaced in the hipping furnace. These precautions, however, can beomitted in the case of a "clean" furnace which has only been used to hiphigh purity silica materials.

After hipping has been completed, various conventional glass treatmentprocedures, such as, annealing, grinding, polishing, drawing, pressing,etc., can be applied to the fully sintered and hipped green body. Theresulting finished product is then ready for use by the consumer.

Based on the foregoing description, the invention will now be furtherillustrated by the following specific examples.

Example 1

This example illustrates the preparation of ultra-pure artificial sandin accordance with the method of the present invention.

21.14 kilograms of TEOS (Eastman Kodak Chemical Company, Rochester, NewYork), was filtered through a 0.6 micron filter (Pall Company, Cortland,N.Y.) into a 50 liter polyethylene container. 9.072 kilograms ofdeionized (DI) water, which had been filtered through a Millipore filter(pore size =0.2 microns), was combined with 0.0209 kilograms ofhydrochloric acid (Fisher Scientific Company, Rochester, N.Y.). Thismixture was also filtered through the Pall filter and then added to theTEOS. The resulting mixture was stirred until a temperature of 65°-75°C. was reached, and then transferred to a quartz reactor and allowed togel. If desired, the mixture can be filtered through a 2.5 micron filterbefore being transferred to the quartz reactor.

Drying and sintering of the gel was performed as follows. First, thereactor was placed in a furnace and the temperature of the furnace wasraised to 100° C. at a rate of 100° C./hour. During this initialheating, the reactor was purged with argon at a rate of 200 cc/minute.The argon, as well as all other gases used in the process, was filteredthrough a 0.6 micron Pall filter before being introduced into thereactor.

The furnace was held at 100° C. for a half an hour, and then raised to300° C. at a rate of 50° C./hour. The argon purging rate was increasedto 800 cc/minute during this second heating phase. When the temperatureof the gel reached 290° C., the furnace was cooled to 250° C. At thispoint, the gel had completely fragmented into fine granules having amean size of less than about 1 millimeter and a surface area to massratio of approximately 200 meter^(2/) gram.

Using a computer-controlled feedback loop, oxygen was then introducedinto the reactor at a rate such that the temperature of the reactor didnot exceed 340° C. as a result of the exothermic reaction between theoxygen and the residual organics associated with the gel. Alternatively,oxygen was introduced into the reactor in accordance with a 10cc/minute/hour ramp up to a maximum of 200 cc/minute. This rate ofoxygen introduction was also found to control the oxygen-organicreaction so as to avoid the formation of charred carbon particles.

The oxygen treatment was continued until the temperature of the geldropped below 300° C., at which point the temperature of the furnace wasramped to 400° C. at a rate of 25° C./hour. The temperature of thefurnace was held at 400° C. for 4 hours. FIG. 1 shows the typicalappearance of the granules at this stage of the process.

After the four hour holding period at 400° C., the temperature of thefurnace was raised to 925° C. at a rate of 75° C./hour to fully sinterthe granules. After the sintering, the granules had 1) a smaller meansize than before sintering, e.g., a mean size on the order of 0.6millimeters, and 2) a reduced surface area to mass ratio on the order of0.08 meter^(2/) gram. FIG. 2 shows the typical appearance of the fullysintered granules.

The overall process from initial preparation of the TEOS solution tofinal sintering of the granules took less than 100 hours.

Flame emission, graphite furnace, and D.C. plasma analyses wereperformed on the fully sintered granules to determine their K, Li, andNa concentrations (flame emission), their Al, Cr, Cu, Fe, Mn, and Niconcentrations (graphite furnace), and their Ba, Ca, Ti, and Zrconcentrations (D.C. plasma). The carbon content of the particles wasdetermined using LECO carbon analysis, and their uranium and thoriumconcentrations were determined by neutron activation analysis. Forcomparison, similar analyses were performed on Deguassa's commercialAEROSIL OX 50 fumed silica (Deguassa Chemical Company, Teterboro, NewJersey).

The results of these analyses are shown in Table II. As shown therein,the artificial sand of the present invention was found to have eitherequivalent purity or to be more pure than fumed silica with respect toeach of the foregoing elements. Moreover, as illustrated in thefollowing example, unlike fumed silica, the artificial sand of thepresent invention is ideally suited for preparing high purity glassarticles by conventional casting techniques.

In addition to the foregoing analyses, neutron activation analyses forforty-one elements were performed on granules which had beenvibra-milled so that they would pass through a 325 mesh screen but notthrough a 600 mesh screen. The results of these analyses are shown inTable III. As shown therein, the granules of the present inventionmaintained their purity through the milling process.

Example 2

This example illustrates the preparation of an ultra-pure, opticalquality glass article from the artificial sand of Example 1 by means ofslip casting.

5,005 grams of the fully sintered granules of Example 1 and 1,407 gramsof DI water were milled in a polyurethane-lined vibra mill with silicamedia for 17 hours. The resulting slurry had a specific gravity of 1.741grams/cc. The particles making up the slurry had a mean size of 12.5microns. The slurry was passed through a 297 micron screen and collectedin a polyethylene container. The container was continuously rotated on aroller mill until slip casting was performed.

A flat plaster of Paris plate was prepared, and a 3" PVC hollow cylinderwas placed onto the top surface of the plate. The inside surface of thehollow cylinder was sprayed with a mold release compound and filled with50 ml of DI water. After the water had been absorbed into the plaster,the cylinder was filled with 125 ml of the artificial sand slurry, carebeing taken not to create bubbles while pouring the slurry into thecylinder.

The cylinder was covered and allowed to stand for 10-15 hours. The greenbody was then removed, covered with a cloth, and allowed to stand atroom temperature for 24 hours prior to drying and partial sintering. Thedensity of the green body was approximately 78% of the final density ofthe finished product.

Drying and partial sintering of the green body was performed in a quartztube furnace as follows. First, the temperature of the furnace wasraised to 110° C. at a rate of 2° C./minute while purging the furnacewith a mixture of argon and oxygen (400 cc/minute argon: 100 cc/minuteoxygen). As in Example 1, the argon and oxygen, as well as the othergases used in processing the green body, were filtered through 0.6micron Pall filters.

The furnace was held at 110° C. for 2 hours and then raised to 1000° C.at a rate of 3° C./minute. After two hours at 1000° C., the argon/oxygenmixture was changed to a helium/chlorine mixture (3.5% chlorine: 200cc/minute flow rate). The temperature of the furnace was then ramped to1150° C. at a rate of 75° C./hour and held at that temperature for 6hours. The helium/chlorine flow rate was then reduced to 25 cc/minute,and the furnace was allowed to cool. When the furnace temperature haddropped into the 200°-300° C. range, the helium/chlorine purge wasstopped and replaced with an argon purge.

The dried and partially sintered green body was fully sintered in atungsten-molybdenum vacuum furnace (vacuum=1×10⁻⁵ torr) as follows. Thebody was placed on zirconia beads and quartz cloth in a molybdenum tray.The tray was placed in the furnace, and the temperature of the furnacewas ramped to 1000° C. at a rate of 25° C./minute. The furnacetemperature was held at 1000° C. for 10 minutes, and then ramped to1450° C. at a rate of 25° C./minute. The furnace was held at 1450° C.for 30 minutes, and then ramped to 1740° C. at a rate of 25° C./minute.After 5-10 minutes at 1740° C., the furnace was allowed to cool to 600°C., at which point the furnace's chamber was backfilled with helium. Thefurnace was then cooled to room temperature, and the fully sinteredgreen body was removed. Although the foregoing processing procedureworks successfully, even better results are achieved when slower rampsare used between the various holding temperatures, e.g., ramps on theorder of 6.5° C./minute.

After sintering, the surfaces of the consolidated body were found to becovered with a thin translucent white haze. This haze was found toresult from the use of zirconia beads as the support for the green body.Use of A-grain zirconia has been found to eliminate the haze. Also, someareas of the body were found to have devitrified. Both the haze and thedevitrified areas were easily removed with hydrofluoric acid. Inaddition to the hydrofluoric acid treatment, the portions of the bodywhich had been in contact with the quartz cloth were ground away.

The fully sintered green body was then hipped using a conventionalhipping furnace as follows. The body was wrapped in Alpha Quartz wooland steel foil and placed in the furnace's pressure chamber. Thepressure in the chamber was raised to 7,000 psi with argon (notfiltered), and then the furnace's temperature was raised to 1320° C. ata rate of 20° C./minute. The pressure in the chamber was raised to20,000 psi and the furnace was held at this pressure and temperature fora period of 45 minutes. Thereafter, the furnace was cooled to 800° C. ata rate of 10° C./minute, and then cooled to room temperature.

To produce the finished product, the hipped, fully-sintered green bodywas annealed, ground, and polished. The annealing was performed asfollows. The body was placed onto a fused silica flat plate and quartzcloth for support and placed in a tube reactor. The temperature in thereactor was raised to approximately 1150° C. at a rate of 240° C./hourand held at this temperature for 1 hour. The reactor was then cooled toroom temperature at a rate of 20° C./hour. Throughout the annealingprocess, the reactor was purged with an argon/oxygen mixture (200cc/minute argon; 50 cc/minute oxygen).

The yield of the various process steps from TEOS polymerization throughto the production of the high density, porous green body were asfollows: TEOS to fully sintered granules--99%; fully sintered granulesto milled granules--97.5%: milled granules to green body--90.3%.Overall, from TEOS to annealed product, the yield was approximately86.3%.

Finished glass products produced in accordance with the foregoingprocedures have been found to have the following characteristics: 1) aparticle count in the glass on the order of 500 counts/cc (this valuecan be reduced by performing the process using conventional clean roomtechniques); 2) an IR transmission coefficient at 2.73 microns of 90-91%for 10 mm of glass: 3) a UV transmission coefficient at 200 nm ofapproximately 82% for 10 mm of glass: 4) a UV transmission coefficientat 185 nm of approximately 70% for 10 mm of glass: 5) a homogeneity inall directions of approximately 2.75×10⁻⁶ for fully annealed samples: 6)a strain point of 993° C.: 7) an annealing point of 1113° C.: 8) anaverage expansion coefficient between 0° C. and 200° C. of 0.54 x 10⁻⁶ ;9) an average expansion coefficient between -100° C. and 200° C. of0.39×10⁻⁶ ; 10) a log₁₀ resistivity of 12.87 ohm-cm at 200° C.: 11) adielectric constant at 1 kHz of 3.91 at 25° C.: 12) a loss tangent at 1kHz of 0.003 at 25° C.: and 13) an Abbe' constant of V_(D) =73.2n_(F)=1.465, n_(S) =1.461, n_(C) =1.459.

For comparison, a commercially available premium quality fused silicaglass sold by Corning Glass Works under the designation 7940 has thefollowing characteristics: 1) a particle count in the glass on the orderof 130 counts/cc for grade AO quality glass: 2) an IR transmissioncoefficient at 2.73 microns of approximately 0% for 10 mm of glass: 3) aUV transmission coefficient at 200 nm of approximately 88% for 10 mm ofglass: 4) a UV transmission coefficient at 185 nm of approximately 80%for 10 mm of glass; 5) a homogeneity in one direction of approximately1.5×10⁶ ; 6) a strain point of 990° C.: 7) an annealing point of 1075°C.; 8) an average expansion coefficient between 0° C. and 200° C. of0.57×10⁻⁶ ; 9) an average expansion coefficient between -100° C. and200° C. of 0.48×10⁻⁶ ; 10) a log₁₀ resistivity of 13.0 ohm-cm at 200°C.: 11) a dielectric constant at 1 kHz of 4.00 at 25° C.: 12) a losstangent at 1 kHz of 0.00002 at 25° C., and 13) an Abbe'constant of V_(D)=67.8, n_(F) =1.463, n_(D) =1.458, n_(C) = 1.456.

Purity data for the glass of the present invention and the 7940 glassare set forth in Table IV.

As these representative data illustrate, the glass produced by thepresent invention is functionally equivalent to the commerciallyavailable premium glass, and indeed is superior to that glass withregard to homogeneity and IR transmission dispersion. With regard topurity, the glass of the invention is generally more pure than the 7940glass. Moreover, the process of the present invention can be used todirectly cast complex shapes, while the commercially available premiumglass is produced in bulk form so that if complex shapes are to be made,they must be machined out of or pressed from sheets of the bulkmaterial.

In sum, as demonstrated by this example, the process of the inventionallows conventional casting procedures to be used to produce highpurity, optical quality glass products which are equivalent to or, withregard to certain properties, superior to existing, commerciallyavailable, premium glass products.

Example 3

This example illustrates the use of the process of the present inventionto prepare the outer portion of the cladding for an optical waveguidefiber.

A conventional chemical vapor deposition process was used to form aglass rod consisting of a central germania-doped region (core)surrounded by a pure silica region (inner portion of the cladding). (Ifdesired, the core can have a uniform index of refraction or can includeregions having different indices of refraction.) The glass rod washeated and stretched to form a segment of "cane" having a diameter inthe 6-8 mm range.

A plaster of Paris mold was formed having an internal cylindrical cavitywhose length was approximately 4 inches and whose diameter wasapproximately 2.4 inches. The walls of the cavity sloped outward fromthe bottom to the top of the cavity at an angle of approximately 1°. Thebottom of the cavity included an aperture for receiving one end of thecane, and the top of the cavity was closed with a plastic cover whichincluded a corresponding aperture for receiving the other end of thecane.

The cane was supported in the mold by means of the top and bottomapertures, the mold was pre-wetted with DI water, and filled with aslurry of milled, artificial sand prepared from TEOS following generallythe procedures described above in Examples 1 and 2. The slurry had aspecific gravity of 1.75.

After 15 hours, a strong green body had formed which was easily removedfrom the mold. The green body was held at room temperature for 48 hours,dried and partially sintered in 1) an oxygen-containing atmosphere and2) a chlorine-containing atmosphere, and then fully sintered undervacuum. The procedures used were generally those described in Example 2.

In some, but not all, cases, the fully sintered green body was hipped,again following generally the procedures of Example 2. Prior to beingdrawn into fiber, the outside surface of the green body wascylindrically ground to a diameter of approximately 45 mm, cleaned withhydrofluoric acid and fire polished. Drawing was performed using astandard draw furnace.

The transmission and strength characteristics of fibers prepared inaccordance with the above procedure are set forth in Table V, wheresamples 1 and 3 were prepared using hipping and sample 2 was preparedwithout hipping. As shown in this table, the fibers had excellenttransmission and strength characteristics. These results are consideredsurprising in view of the fact that they represent initial experiments,as opposed to a fully refined and optimized process.

                  TABLE I                                                         ______________________________________                                        Water:TEOS                                                                    Mole Ratio   5 to 1     10 to 1    15 to 1                                    ______________________________________                                        Sintering           Surface Area                                              Temperature (°C.)                                                                          (M.sup.2 /gram)                                           400          192        596        673                                        500          150        560        663                                        600          122        475        548                                        700          118        398        443                                        800           13        206        354                                        900          <0.5        4          88                                        950          --         --         <0.5                                       Density before                                                                             1.529      --         1.290                                      sintering                                                                     (gm/cm.sup.3)                                                                 ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        PURITY (ppb)                                                                                Deguassa Granules of the                                        Element       OX-50    Present Invention                                      ______________________________________                                        Carbon (%)    *        0.034                                                  Lithium       27       <54                                                    Sodium        <233     <233                                                   Potassium     <966     <966                                                   Barium        <110     <110                                                   Calcium       200      150                                                    Titanium      6140     220                                                    Zirconium     240      230                                                    Aluminum      66500**  <23                                                    Chromium      51       1                                                      Copper        5        3                                                      Iron          927      46                                                     Manganese     10       <4                                                     Nickel        55       <18                                                    ______________________________________                                         *-not measured                                                                **Aluminum analysis performed by the D.C. plasma technique.              

                  TABLE III                                                       ______________________________________                                        Neutron Activation Analysis of Milled Silica Granules                         (parts per million)                                                           ______________________________________                                        Titanium      <35.0                                                           Tin           <0.5                                                            Iodine        <0.003                                                          Manganese     0.322 ± 5.0%                                                 Copper        <0.6                                                            Vanadium      <0.02                                                           Chlorine      <0.3                                                            Aluminum      20.937 ± 1.0%                                                Mercury       <0.0015                                                         Samarium      <0.0001                                                         Tungsten      <0.7                                                            Molybdenum    <0.009                                                          Uranium       <0.0001                                                         Lanthanum     <0.0004                                                         Cadmium       <0.017                                                          Arsenic       <0.001                                                          Antimony      <0.0001                                                         Zirconium     <0.4                                                            Bromine       0.0038 ± 15.0%                                               Sodium        0.259 ± 10.0%                                                Potassium     <100.0                                                          Cerium        0.0015                                                          Calcium       <5.0                                                            Lutetium      <0.001                                                          Europium      <0.005                                                          Selenium      <0.010                                                          Terbium       <0.0006                                                         Thorium       <0.0005                                                         Chromium      0.002 ± 20.0%                                                Ytterbium     <0.004                                                          Hafnium       <0.001                                                          Barium        <0.1                                                            Neodymium     0.013 ± 20.0%                                                Cesium        <0.0005                                                         Silver        <0.002                                                          Nickel        <0.45                                                           Scandium      0.00003 ± 15.0%                                              Rubidium      <0.005                                                          Iron          0.258 ± 20.0%                                                Zinc          <0.025                                                          Cobalt        0.0017 ± 12.0%                                               ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        PURITY (ppb)                                                                  Element      7940       Present Invention                                     ______________________________________                                        Sodium       <233       <233                                                  Potassium    <966       <966                                                  Copper       1521       2-7                                                   Magnesium    10-100     *                                                     Calcium      180        350-380                                               Zinc         50-500     *                                                     Boron        50-500     *                                                     Aluminum     <32        <25                                                   Chlorine     10000-100000                                                                             *                                                     Titanium     420        120-180                                               Phosphorous  10-100     *                                                     Arsenic      1-5        *                                                     Antimony     1-5        *                                                     Bismuth      10-100     *                                                     Vanadium     10-100     *                                                     Chromium     4          1-<24                                                 Manganese    <4         <3                                                    Iron         117        42-48                                                 Lithium      <54        <54                                                   Nickel       <20        2-<15                                                 Zirconium    170        180-230                                               Barium       <100       <110                                                  Uranium      *          .1-.3                                                 Thorium      *          <.5                                                   ______________________________________                                         *not measured                                                            

                  TABLE V                                                         ______________________________________                                                                 Fiber Strength                                               Attenuation Rate Weibull 50%                                          Sample  (dB/Km)@1300 nm  Failure Probability                                  ______________________________________                                        1       0.39-0.50        60        KPSI                                       2       0.36-0.50        426.9     KPSI                                       3       0.39             426.6     KPSI                                       ______________________________________                                    

What is claimed is:
 1. A method for consolidating a green bodycomprising the steps of:(a) drying and partially sintering the greenbody in a chamber by:(i) raising the temperature of the chamber to aboveabout 1000° C.: and (ii) introducing chlorine into the chamber during atleast a portion of said drying and partial sintering to reduce the levelof bound water associated with the green body: (b) fully sintering thegreen body in a chamber by raising the temperature of the chamber to atemperature above about 1720° C. while applying a vacuum to the chamber;and (c) hot isostatic pressing the fully sintered green body in achamber by raising the temperature of the chamber to above about 1150°C. and introducing an inert gas into the chamber at a pressure aboveabout 100 psig.
 2. The method of claim 1 wherein oxygen is introducedinto the chamber during at least a portion of step (a) to reduce thelevel of organic material associated with the green body.
 3. The methodof claim 1 wherein the fully sintered green body is wrapped in glasswool prior to step (c).
 4. The method of claim 1 wherein the green bodyis composed of silica.